1. Field of the Invention
The present invention relates to a Laser Induced Thermal Imaging (LITI) mask and an organic electroluminescent device fabrication method using the mask. More particularly, the present invention relates to a LITI mask in which corner regions are reinforced, and an organic electroluminescent device fabrication method using the mask.
2. Description of the Related Art
An organic electroluminescent device in flat panel displays has merit as a moving picture display media irrespective of the size of the display since the organic electroluminescent device has a high response speed of 1 ms or less, consumes less electrical power than other devices and does not have a viewing angle problem due to its self-light emitting characteristics. Furthermore, the organic electroluminescent device is being considered as a next generation flat panel display because it can be fabricated at a low temperature and is simply fabricated based on an existing semiconductor process technology.
A patterning technology of an organic film composing the organic electroluminescent device is important in the organic electroluminescent device. In the organic layer patterning technology, the Laser Induced Thermal Imaging (LITI) method has excellent pattern uniformity and is advantageous in scaling-up of the organic film.
The LITI method has excellent inner uniformity of pixels when the organic film is scaled up since spin coating characteristics are used in the LITI method as is. Furthermore, the LITI method is capable of solving a problem of life deterioration of device by preventing damage of the organic film due to a solvent since the LITI method is not a wet process, but is rather a dry process. Additionally, the LITI method has merit in that the organic film can be finely patterned.
In order to apply the LITI method, a light source and an organic electroluminescent device substrate, i.e., a substrate and a donor substrate, are required, and the donor substrate includes a substrate layer, a light to heat conversion layer and a transfer layer. The LITI method includes a step of absorbing light coming out of the light source into the light to heat conversion layer of the donor substrate such that light is converted into thermal energy, and a step of transferring an organic material formed on the transfer layer by the converted thermal energy to the substrate such that the organic film is formed.
In a process of forming organic layer patterns by a LITI method, a donor substrate is laminated with a receptor substrate on which a prescribed device is formed; the donor substrate includes a substrate layer, a light to heat conversion layer and a transfer layer formed of a material composing an organic layer to be patterned. The receptor substrate sequentially includes a substrate, a pixel electrode pattern and a pixel defining layer; the pixel defining layer includes an opening for exposing a prescribed portion of the pixel electrode pattern.
The organic film is patterned by transferring an organic film using a laser such that the organic film is separated from a portion which does not receive laser energy while the organic film is being transferred to a receptor substrate after an organic film adhered to the substrate comes off from a donor substrate by the action of laser energy passing through a mask having a shape to be patterned.
The transfer characteristics are determined by a first adhesive force between the receptor substrate and a transfer layer a cohesive force between the transfer layers and a second adhesive force between the transfer layer and a substrate; the cohesive force between the transfer layers should be lower than adhesive force between the respective substrates and the transfer layer in order to improve laser transfer characteristics.
While a transfer layer of a region in which a laser beam is irradiated on the donor substrate is reaching a receptor substrate, the transfer layer comes off due to the degree of ductility of the donor substrate and the surface level difference of the receptor substrate; the transfer layer comes off severely particularly on a portion including an apex on one side of the receptor substrate. A contact area of the receptor substrate and the transfer layer separated from the donor substrate is small on the one portion including an apex on one side of the receptor substrate compared with other regions. Therefore, on the one portion A including an apex on one side of the receptor substrate, the foregoing second adhesive force, that is, adhesive force between the receptor substrate and the transfer layer is deteriorated.
The cohesive force between the transfer layers can be higher than the first adhesive force on the portion including an apex on one side of the receptor substrate during a process of adsorbing the donor substrate from the receptor substrate after being irradiated by a laser beam. Therefore, an organic layer pattern is unevenly formed on a portion A including an apex on one side of a pixel portion to result in deterioration of picture quality or the generation of an opening defect in which a prescribed portion of the pixel portion is open on a portion of the organic layer pattern.
A photolithography mask equipped with a serif for solving the rounding defect when a rounding defect is formed on a semiconductor integrated circuit pattern is suggested in U.S. Pat. No. 5,707,765. However, the photolithography mask discussed therein is equipped with a serif that is a reinforced region on the edge of a mask pattern to supplement rounding of the pattern caused by an optical problem generated during a fine patterning process, i.e., a proximity effect generated on a portion of pattern due to light diffraction. However, there is a difference on the design aspect when solving an opening defect formed since the adhesive force of the edge is deteriorated by a surface level difference of the receptor substrate during a process of forming an organic layer pattern by the LITI method.